1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, more particularly to an electrophotographic photosensitive member having a photosensitive layer excellent in durability without image quality deterioration by repetition.
2. Related Background Art
In recent years, a large number of electrophotographic photosensitive members having organic compounds as the photoconductor have been developed.
Among them, most of those practically applied have the form of a photoconductor which is functionally separated into a charge generation material and a charge transport material.
An electrophotographic photosensitive member by use of such organic photoconductor is expected to be further improved in electrophotographic characteristics such as sensitivity, light response, etc., due to flexibility of material design, and also it is characterized by easy film forming property and high productivity.
Whereas, an electrophotographic photosensitive member is subject to various image forming processes repeatedly in an electrophotographic device, and during that period, it is required to exhibit stable characteristics. However, the electrophotographic photosensitive member by use of the organic photoconductor as described above has the drawback that lowering in image density accompanied with lowering in charging ability and image quality deterioration such as blurring of the image accompanied with lowering in surface resistance are liable to occur after repeated use.
The cause for these deteriorations may be considered to be the large influence from corona charging.
More specifically, when a photosensitive member is used in a copying machine, it is constantly exposed to an atmosphere of corona discharging, and the organic photoconductor may be considered to be subjected to deterioration by the active species such as ozone formed by corona discharging as copying is performed repeatedly.
Particularly, in an electrophotographic photosensitive member having an organic photoconductor, which is frequently used with negative charging, more amount of ozone is generated than in the case of positive charging, and this is also considered as one factor more susceptible to deterioration as compared with other photosensitive members by use of positive charging.
In the prior art, as the method for preventing such deterioration of electrophotographic photosensitive member, it has been proposed to use various antioxidants such as dilaurylthiodipropionate as disclosed in Japanese Laid-open Patent Publication No. 48-40436, 2,4,6-trialkylphenol derivatives as disclosed in Japanese Laid-open Patent Publication No. 48-75241; hindered amine derivatives as disclosed in Japanese Laid-open Patent Publication No. 50-44835; phenol type compounds, hydroxyanisole type compounds, hydroquinone type compounds, sulfur compounds, organic phosphorus compounds and p-phenylenediamine compounds as disclosed in Japanese Laid-open Patent Publications Nos. 56-130759 and 57-122444; benztriazole derivatives as disclosed in Japanese Laid-open Patent Publication No. 58-120260; hydroxy aromatic compounds as disclosed in U.S. Pat. No. 4,563,408 (corresponding to Japanese Laid-open Patent Publication No. 61-156131); hindered phenol type compounds as disclosed in Japanese Laid-open Patent Publication No. 62-105151; and so on.
By addition of these various antioxidants, deterioration of the photosensitive member can be prevented to some extent. However, practically, further improved prevention effect is required, and at the same time, it would be desirable to have an improved method without causing such problems that these antioxidants themselves act as the trapping agent for migration of the charges to give rise to elevation in light potential (V.sub.L).